DIDELPHYS 2 713 
sent the same laterally compressed shape as in the fossil. 
In addition to the perfect tooth, the fossil includes the 
empty sockets of two other teeth, and the relative position 
of these sockets places the Perameles out of the pale of 
comparison. On the hypothesis that the present fossil 
represents a species of Didelphys, the tooth im sitw unques- 
tionably corresponds with the second, or middle false molar, 
right side, lower jaw. This is proved by the size and 
position of the anterior alveolus. 
Had the tooth a situ been the one immediately pre- 
ceding the true molars, the socket anterior to it should 
have been at least of equal size, and in juxta-position to 
the one containing the tooth. The anterior socket, how- 
ever, is little more than half the size of the one in which 
the tooth is lodged ; it is, also, separated from that socket 
by an interspace, equal to that which separates the first 
from the second false molar in the Didelphys Virginiana. 
This is well shown in the inside view. In the placental 
Mammalia, in which the first small false molar is similarly 
separated by a diastema from the second, the first false 
molar has only a single fang. In the present fossil, the 
empty socket of the first false molar proves that the tooth 
had two fangs, as in the marsupial Pere and Lnsectivora. 
There is nothing in the size or form of the socket, posterior 
to the implanted tooth of the fossil, to forbid the supposi- 
tion that it contained a false molar, resembling the one 
in place; had it been the socket of a true molar, then the 
fossil could not have belonged to Didelphys, or to any other 
known marsupial genus, because no known marsupial ani- 
mal, which presents the posterior false molar of a similar 
form and in like juxta-position with the true molars, as the 
tooth in the present fossil, (on the supposition that it im- 
mediately preceded the true molars,) has the next false 
